This invention relates generally to air separation units, and more particularly, to a method of designing or building air separation units by using libraries containing different module designs, and to air separation units constructed according to such a method.
FIG. 1 is a schematic illustration of a portion of an air separation plant or unit 10 for the production of oxygen, nitrogen and/or argon. A main air compressor (MAC) 11 is used to produce a compressed air stream, e.g., at a pressure of 5-6 atmospheres, which is fed to pre-purification units (PPU) 12a-b in which carbon dioxide, water, trace hydrocarbons and other condensable substances are removed.
The air leaving the PPU 12a-b is then split into two streams. A main air stream 13a, comprising between about 40% to about 80% of the air volume leaving the PPU, is fed to main heat exchanger (MHE) 14. Second stream 13b is passed to booster compressor 15 to produce a boosted air stream 13c having a pressure of about 10-70 atmospheres. The split ratio between the main air stream and the boosted air stream is a factor of a number of variables not the least of which is the desired product mix of the air separation plant. The boosted air is also fed to the MHE.
The MHE internals are of standard design, and the MHE is operated in standard fashion. The main air exits the MHE as saturated vaporous main air stream 13d at a temperature of about xe2x88x92187 C. (about xe2x88x92280 F.). The boosted air exits the MHE as liquid boosted air stream 13e at a temperature of less than about xe2x88x92187 C.
In certain air separation plant designs, main air side stream 13f is withdrawn from main air 13a, and passed through the MHE. The temperature of the main air side stream 13f is lowered within the MHE to about xe2x88x92140 C. (xe2x88x92220 F.), and it is withdrawn as vaporous main air side stream 13g. This side stream is then passed through expander 16 to lower its temperature to less than about xe2x88x92180 C. (xe2x88x92292 F.), and then fed as expanded main air side stream 13h directly into low pressure column 22.
Both the vaporous main air and liquid boosted air streams 13d and 13e are then fed to high pressure column (HPC) 17 at about xe2x88x92187 C. for separation into an oxygen-enriched liquid stream and a nitrogen-enriched product. The gaseous nitrogen product stream 32 may be withdrawn from the upper section of the HPC at about xe2x88x92200 C. (about xe2x88x92300 F.).
The HPC internals are of any standard construction, e.g., structured packing, distillation trays, etc., and the HPC is operated in a conventional fashion. An oxygen-rich liquid (RL) stream 18 is withdrawn from the lower section of the HPC 17, and comprises about 30-45% by volume oxygen, with the remainder being nitrogen, argon and residual air components such as xenon, krypton, and so on. Poor liquid (PL) stream 29 comprising essentially nitrogen, with various residual air components such as neon, etc., is withdrawn from the top section of the HPC 17. The split of these two liquid streams 18 and 29 is typically 55% by volume (or mole %) rich liquid and 45% by volume (or mole %) poor liquid. Both the rich and the poor liquid streams 18 and 29 are fed separately to subcooler 20, which is a refrigeration recovery heat exchange unit.
The resulting subcooled streams 21a-b are fed to low pressure column (LPC) 22, which is typically located above and is thermally coupled with the HPC 17. These subcooled streams enter the LPC 22 at a temperature of about xe2x88x92207 C. (about xe2x88x92316 F.) and at a pressure of between about 1 and 2 atmospheres. At this temperature and pressure, liquid oxygen 23a collects at the bottom of the LPC 22 from where liquid oxygen product stream 23b is withdrawn. The purity of the liquid oxygen product stream can vary from about 95% or less oxygen (low purity oxygen) up to and in excess of 99.9% oxygen (high purity oxygen). The actual purity or composition of the liquid oxygen stream depends in large part upon the manner in which other parts of the air separation plant are operated. If desired, another liquid stream 30 (also known as xe2x80x9cintermediate liquidxe2x80x9d stream) may be withdrawn from the HPC 17 and fed to the LPC 22 as an additional reflux stream.
Liquid oxygen generated in the lower section of the LPC 22 passes to the reboiler-condenser (R-C) 26, a portion of which is submerged within liquid oxygen in sump 23a. Gaseous nitrogen 27a from the HPC 17 is condensed in R-C 26 by indirect heat exchange with liquid oxygen in sump 23a, resulting in partially reboiling of the liquid oxygen. The condensed nitrogen stream 27b from R-C 26 enters the HPC 17. A poor liquid stream 29 is withdrawn from the HPC 17 and is passed to subcooler 20. This poor liquid stream can be withdrawn either from the same point as nitrogen stream 27b or it may be withdrawn several stages below the nitrogen stream 27b feedpoint.
The liquid oxygen product stream 23b withdrawn from the sump 23a at a pressure of about 1 atmosphere can be transferred either directly, or optionally via subcooler 20, to a liquid oxygen (LOX) storage tank 32, and optionally, via subcooler 20. If desired, oxygen can be withdrawn from the LOX storage tank 32, and pressurized to about 5-70 atmospheres by a liquid oxygen pump P24. The pressurized LOX product stream then exchanges heat with the main and boosted air streams 13a and 13c in the MHE 14, resulting in the formation of gaseous oxygen product stream 23c, which is recovered at a pressure of about 5-70 atmospheres.
Gaseous nitrogen waste stream 25a from the upper section of the LPC 22 is returned to the subcooler 20 for heat exchange with the rich and poor liquid streams from the HPC 17, and then discharged from the air separation unit after additional heat exchange with the main and boosted air streams in the MHE 14. Optionally, a product nitrogen stream (not shown) may also be withdrawn from the LPC 22 and recovered as a nitrogen product after undergoing heat exchange in subcooler 20 and MHE 14.
Aside from the configuration shown in FIG. 1, many other variations are also possible. Depending on the capacity requirement, different designs for the MHE is available with a different number of heat exchanger cores, e.g., 2-10 or more cores. If argon product is desired, then another distillation column may be coupled to the LPC 22 for additional processing of a fluid stream withdrawn from LPC 22. Moreover, the columns, compressors, expanders and other equipment can be arranged differently from that shown in FIG. 1, depending upon the refrigeration requirements of the plant. Labels A-N represent interface connections that will be addressed in conjunction with the discussion of FIG. 4.
Applications requiring smaller quantities, e.g., less than 200 metric tons per day (MTPD), of oxygen or nitrogen will usually require only one product and the specifications for that product (e.g., purity, pressure, flows, etc.) are well known. The commercial solution for these applications is typically building a small pre-fabricated plant, e.g., a nitrogen generator, and installing it on the customer""s site (known as an xe2x80x9con-site applicationxe2x80x9d). The economic drivers for plants of this nature are low cost, repeatability, small xe2x80x9cfootprintxe2x80x9d and the like. These plants are highly standardized, and all major air separation unit suppliers have plants of this nature in one form or another.
Applications requiring larger quantities of gas, e.g., 200-2000 or more MTPD, sometimes require more than one product and the specifications for these products vary significantly. The commercial solution for these applications typically is a scheme in which large quantities of the desired gas are piped xe2x80x9cover the fencexe2x80x9d from a production facility located next to the customer""s plant. In this way, the delivered gas is metered in much the same way as any other utilities (e.g., electricity, natural gas, water, etc.). The economic drivers for solutions to these plants include low evaluated cost, i.e., capital and power, robustness of the design, minimized footprint, and the like. These requirements are usually met by either repeating an existing plant design with specifications similar to those desired, or engineering a new design specific to the needs of the given application (known as a xe2x80x9ccustomxe2x80x9d or xe2x80x9cengineered-to-orderxe2x80x9d plant).
All air separation plant suppliers use variants of these two approaches to achieve an economical solution for a given application. If a repeat design is available, then that design is generally the best solution since the engineering effort does not require duplication (almost by definition, only minimal adjustments are required). However, rarely is a repeat design available because, for larger plants at least, the specifications and economic drivers usually differ significantly from plant to plant.
In those situations in which a repeat design is not available, the plant supplier typically begins with an existing plant design that is closest to the required specifications and then modifies that design to fit the specifications. Although there may be relatively few changes to the fundamental designs of the plant, typically a large number of changes are required if only as a result of changes in size of various components of the plant. Moreover, any change (no matter how small) will have a significant impact on other parts of the plant, i.e., it creates a xe2x80x9cdominoxe2x80x9d or xe2x80x9cripplexe2x80x9d effect. In particular, the spatial coordinates of inlets and outlets to various components, e.g., columns and heat exchangers, can necessitate a complete redesign of all of the pipe-work, an onerous and time consuming task given the complexity of an air separation plant.
Of particular interest to the economic solution of air separation plants of this size, particularly of a size between about 200-2000 MTPD, is an approach with unique design features that achieve the benefits of standardization (e.g., repeatability, reduced risk, supplier leverage, etc.) while at the same time achieving the benefits of a customized design (e.g., better fit, better balance between power and capital cost optimization, etc.).
One aspect of the invention provides for a method of designing an air separation unit. The method involves selecting a first module from a first library containing at least two different designs of the first module, selecting a second module from a second module library containing at least two different designs of the second module. Each of the module designs in the first library has a first set of interface points with substantially the same relative spatial coordinates as every other design in that library, and each of the module designs in the second library has a second set of interface points with substantially the same relative spatial coordinates as every other design in the second library.
According to another aspect of the invention, the method involves selecting a high pressure column module from a first library for coupling to a main heat exchanger module from a second library and for coupling to a low pressure column module from a third library. The first library contains different high pressure column modules, with each module having a first and second set of interface points, and each of the two sets of interface points has relative spatial coordinates that are substantially the same as every other high pressure column module in the first library. The second library contains different main heat exchanger modules, with each module having a third set of interface points with relative spatial coordinates that are substantially the same as every other main heat exchanger module in the second library, and the third set of interface points are designed for coupling to the first set of interface points on each of the high pressure column modules. This method further involves providing a third library containing different low pressure column modules, with each module having a fourth set of interface points with relative spatial coordinates that are substantially the same as every other low pressure column module in this library, and the fourth set of interface points are designed for coupling to the second set of interface points on each of the high pressure column modules.
Another aspect of the invention relates to a method of building an air separation plant based on a design obtained according to a design method. The design method involves selecting a main heat exchange module design and a high pressure column module design from two separate libraries, one of which contains different main heat exchanger module designs and the other contains different high pressure column module designs. Each of the different heat exchange module designs has a first set of predetermined interface points for connecting to a second set of predetermined interface points on each of the different high pressure column module designs, with the first set of predetermined interface points having the same relative spatial coordinates, and the second set of predetermined interface points having the same relative spatial coordinates.
Another aspect of the invention provides for an air separation unit having a first module with a first set of interface points coupled to a second set of interface points of a second module. The first module is selected from a first library containing different designs of the first module, with each of the different designs having at least some interface points that have substantially the same relative spatial coordinates as the first set of interface points. The second module is selected from a second library containing different designs of the second module, with each of the different designs having at least some interface points that have substantially the same relative spatial coordinates as the second set of interface points.
Yet another aspect of the invention provides for a library for use in designing an air separation unit, with the library containing at least two modules and each of the modules has substantially the same relative spatial coordinates of interface points as every other module in the library.